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REFRACTORIES                                       505
Grog clay refractories are almost invariably made by hand with or without the use of molds, though recently the manufacture of pots and other products by the process of casting in plaster molds has been begun. This method consists in preparing a viscous suspension of the clay and grog in water, to which carbonate and silicate of soda have been added in amounts up to 0.4 per cent of the dry weight of the mixture. The best proportion of these reagents seems to be 3 parts of the carbonate to 4 parts of the silicate. These alkaline reagents cause the water content to be not more than that of the plastic mass. The fluid mixture, called slip, is then poured into plaster molds with or without a core, where it is allowed to remain until the desired thickness of wall has formed when the excess is removed. In core molds the casting is solid and the core must be removed before the mass begins to shrink.
Uses of Clay Refractories.—Some of the uses of clay refractories may be indicated as follows: High clay materials, high grade: Blast-furnace linings and stoves, heating furnaces, open-hearth furnaces, soaking pits, boiler installations, ntoker arches, muffle furnaces, crucible furnaces, cupolas, malleable-iron furnaces, rotary cement kilns, enameling furnaces, side walls of glass tanks, pot furnaces, carbon furnaces, ceramic kilns, gas producers, sintering and roasting furnaces, gas installations, regenerators, recuperators, reverberatory furnaces, lime kilns, bullion, dross, brass, lead, zinc and copper furnaces, etc.
Intermediate and low-grade clay refractories: Annealing furnaces, runner brick, sleeves and nozzles, ladles, oil refineries, stills, incinerators, vaults, flues, furnace HttickH, digesters, bake ovens, dryers, carburetors, etc.
Siliceous refractories, high grade: Beehive coke ovens, gas benches, furnace arches, boiler installations, ceramic kilns, regenerators and many uses given for high-grade materials.
(Jrog refractories: Glass pots, tank blocks, glass-house accessories, muffles, special shapes for all kinds of furnaces, zinc retorts, etc.
SILICA REFRACTORIES
Lime Bond Refractories.—The materials of this class consist largely of crushed quarUite bonded together with a small amount, usually about 2 per cent, of CaO in the form of hydratod lime. For some purposes blocks of the natural quartzite or quart/ schist arc employed. There are three principal sources of quartzite suitable for this purpose in the United States, the Tuscarora or Medina formation in Blair and Huntingdon Counties, Pennsylvania, the Baraboo formation, in the Devil's Lake Region, Wisconsin, and the Weisner formation (Lower Cambrian), near Birmingham, Alabama. In addition, quartzite is obtained for this purpose in Montana, from the Quadrant formation (Philipsburg, Mont.)- In certain localities, as at Pueblo, Colo., and in Fayette County, Pennsylvania., quartz sandstones an; used, and attempts have been made to employ Indiana chert, from the Mitchell Limestone formation.
The quarlzites to he suitable for this purpose must be of a character which permits of the transformation of the quartz to cristobalite under heat treatment of reasonable duration and which must retain a structure of sufficient mechanical strength. Both requirements arc essential. Materials like chert transform quite readily to erintolmlite hut, are deficient in final strength. Quartzites which fail to show a transformation rale, commercially applicable, or are lacking in strength, must be rejected for this purpose. Although all silica refractories show an increase in permanent volume after reheating this further expansion must be kept within definite limits, '}• ui to *£ in. per foot or 1.6 to 2.1 per cent of the original length.